In some communication systems, a receiver (and/or a transmitter) may be coupled to a plurality of transmitting (and/or receiving) devices via a shared communication network. The communication network may be shared, for example, by using OFDMA (orthogonal frequency division multiple access), a multi-user variant of OFDM (orthogonal frequency division multiplexing). In OFDM, digital data, that may be encoded, is modulated onto a plurality of orthogonal subcarrier frequencies (“subcarriers”). Modulation schemes may include, for example, QAM (quadrature amplitude modulation) and/or PSK (phase shift keying). Subcarriers may further include so-called “pilots” that are subcarriers modulated with known data that may then be used for determining channel characteristics.
In OFDMA, each client device of a plurality of client devices may utilize (e.g., may be assigned) a subset of the subcarriers. The client device (transmitting or receiving) may then be configured to modulate user data onto the subset of subcarriers and transform the modulated subcarriers (and pilots) into the time domain (using, e.g., an inverse fast Fourier transform) to produce an OFDM symbol. The OFDM symbol may then be converted from digital to analog, modulated onto an RF carrier (for example) to produce an analog signal and transmitted along a channel to a receiving device. The receiving device may be configured to demodulate the analog signal and digitize the demodulated analog signal (via, e.g., an analog to digital converter) to recover the OFDM symbol. The OFDM symbol may then be further processed to recover the user data.
The analog signal may be modified during its travel along the channel due to channel characteristics that may change over time. Channel characteristics may include channel frequency response and/or channel impulse response. In a multi-user system, channel characteristics may vary across channels since a first communication channel that couples a first client device to a head-end may be different from a second communication channel that couples a second client device to the head-end.
For example, in a broadband cable system, a cable modem termination system (“head-end”) is configured to provide television, voice and network access to a plurality of cable modems over a hybrid fiber coaxial cable network. A configuration of each channel coupling a respective cable modem to the head-end may vary across channels. Thus, channel characteristics may vary across channels. Channel characteristics may be estimated using periodic probing symbols. Each channel characteristic estimate may be applied to data-carrying OFDM symbols until a next probing symbol is received and the channel characteristic estimate is updated. The probing symbols may be transmitted relatively far apart, e.g., at intervals ranging from about one second to about thirty seconds. Thus, several data-carrying OFDM symbols may be transmitted (and received) between probing symbols.
The channel characteristics are assumed to be static, i.e., to remain constant between probing symbols. However, channel characteristics may actually change over the time interval between probing symbols. Further, small timing drifts may appear as changes in the channel frequency response. Thus, the assumption that the channel characteristics are fixed between probing symbols may result in a loss of performance as equalization may be performed on data OFDM symbols using channel estimates that are no longer accurate.
Generally, in a receiving device successive received OFDM symbols are separated (in time) by a guard interval. The guard interval is configured to accommodate echoes or micro-reflections from, e.g., discontinuities in the channel. The guard interval may include a cyclic prefix used to facilitate performing a discrete Fourier transform (e.g., a fast Fourier transform (FFT)) utilized in recovery of data from an OFDM symbol. Further, communication channels may be susceptible to in-band interference. Typically, in-band interference is not orthogonal to OFDM sub-carriers so that when the FFT is performed, a narrow band interferer may experience spectral widening due to the windowing operation associated with the FFT. In other words, the window length is not typically a whole number multiple of interferer frequency periods. Thus, an external interferer with a bandwidth of a few MHz (Megahertz) may spread to tens of MHz because of the windowing, thereby affecting a number of sub-carriers and their associated data.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art.